Bulk non-planar transistor having strained enhanced mobility and methods of fabrication

ABSTRACT

A method of a bulk tri-gate transistor having stained enhanced mobility and its method of fabrication. The present invention is a nonplanar transistor having a strained enhanced mobility and its method of fabrication. The transistor has a semiconductor body formed on a semiconductor substrate wherein the semiconductor body has a top surface on laterally opposite sidewalls. A semiconductor capping layer is formed on the top surface and on the sidewalls of the semiconductor body. A gate dielectric layer is formed on the semiconductor capping layer on the top surface of a semiconductor body and is formed on the capping layer on the sidewalls of the semiconductor body. A gate electrode having a pair of laterally opposite sidewalls is formed on and around the gate dielectric layer. A pair of source/drain regions are formed in the semiconductor body on opposite sides of the gate electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of integrated circuitmanufacturing and more particularly to the formation of a strainenhanced mobility bulk nonplanar transistor and its method offabrication.

2. Discussion of Related Art

Modern integrated circuits, such as microprocessors, are made up ofliterally hundreds of millions of transistors coupled together. In orderto improve the performance and power of integrated circuits, newtransistor structures have been proposed. A nonplanar transistor, suchas a tri-gate transistor, has been proposed to improve deviceperformance. A tri-gate transistor 100 is illustrated in FIGS. 1A and1B. FIG. 1A is an illustration of a overhead/side view of a tri-gatetransistor 100 and FIG. 1B is an illustration of a cross-sectional viewtaken through the gate electrode of a tri-gate transistor 100. Tri-gatetransistor 100 includes a silicon body 102 having a pair of laterallyopposite sidewalls 103 and a top surface 104. Silicon body 102 is formedon an insulating substrate including an oxide layer 106 which in turn isformed on a monocrystalline silicon substrate 108. A gate dielectric 110is formed on the top surface 104 and on the sidewalls 103 of siliconbody 102. A gate electrode 120 is formed on the gate dielectric layer110 and surrounds the silicon body 102. A pair of source/drain regions130 are formed in the silicon body 102 along laterally oppositesidewalls of gate electrode 120. Transistor 130 can be said to be atri-gate transistor because it essentially has three gates (G₁, G₂, G₃)which essentially form three transistors. Tri-gate transistor 100 has afirst gate/transistor on one side 103 of silicon body 102, a secondgate/transistor on a top surface 104 of silicon body 102 and a thirdgate/transistor on the second side 103 of silicon body 102. Eachtransistor provides current flow proportional to the sides of siliconbody 102. The tri-gate transistor are attractive because they have largecurrent per area which improves device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an overhead view of a standard tri-gate transistor.

FIG. 1B shows a cross-sectional view of standard tri-gate transistor.

FIG. 2 is an illustration of a bulk tri-gate transistor having a straininduced mobility in accordance with an embodiment with the presentinvention.

FIGS. 3A–3I illustrate a method of forming a bulk tri-gate transistorhaving a strain enhanced mobility in accordance with an embodiment ofthe present invention.

FIGS. 4A–4C illustrate a method of forming a bulk tri-gate transistorhaving a strain enhanced mobility in accordance with an embodiment ofthe present invention.

FIG. 5 illustrates crystal lattices for a bulk silicon, a strainedsilicon germanium semiconductor body and a stained silicon cappinglayer.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Embodiments of the present invention are bulk nonplanar transistorshaving strained enhanced mobility and their methods of fabrication. Inthe following description, numerous specific details have been set forthin order to provide a thorough understanding of the present invention.In other instances, well known semiconductor processing and fabricationtechniques have not been set forth in particular detail in order to notunnecessarily obscure the present invention.

Embodiments of the present invention are bulk nonplanar transistorshaving strained enhanced mobility and their methods of fabrication.Embodiments of the present invention include a semiconductor body whichplaces a capping layer formed on or around the semiconductor body understrain. A capping layer under strain increases the mobility of carriersin the device which increases the current of the device which can beused to improve circuit speeds.

An example of a bulk nonplanar or tri-gate transistor 200 having strainenhanced mobility is illustrated in FIG. 2. Transistor 200 is formed ona bulk semiconductor substrate 202. In an embodiment of the presentinvention, the substrate 202 is a monocrystalline silicon substrate.Formed in semiconductor substrate 202 are a pair of spaced apartisolation regions 204, such as shallow trench isolation (STI) regions,which define the substrate active region 206 therebetween. Substrate202, however, need not necessarily be a silicon monocrystallinesubstrate and can be other types of substrates, such as but not limitedto germanium (Ge), silicon germanium (Si_(x)Ge_(y)), gallium arsenide(GaAs), InSb, GaP, and GaSb. The active region 206 is typically doped toa p type conductivity level between 1×10¹⁶ to 1×10¹⁹ atoms/cm³ for an ntype device and doped to an n type conductivity level between 1×10¹⁶ to1×10¹⁹ atoms/cm³ for a p type device. In other embodiments of thepresent invention, the active region 206 can be an undopedsemiconductor, such as an intrinsic or undoped silicon monocrystallinesubstrate.

Transistor 200 has a semiconductor body 208 formed on active substrateregion 206 of bulk substrate 202. The semiconductor body 208 has a topsurface 209 and a pair of laterally opposite sidewalls 211. The topsurface 209 is separated from the bottom surface formed on semiconductorsubstrate 206 by a distance which defines the body height. The laterallyopposite sidewalls 211 of the semiconductor body 208 are separated by adistance which defines the body width. The semiconductor body 208 is amonocrystalline or single crystalline semiconductor film. In anembodiment of the present invention, the semiconductor body 208 isformed from a semiconductor material different than the semiconductorused to form the bulk substrate 202. In an embodiment of the presentinvention, the semiconductor body 208 is formed from a singlecrystalline semiconductor having a different lattice constant or sizethan the bulk semiconductor substrate 202 so that the semiconductor body208 is placed under strain. In an embodiment of the present invention,the bulk semiconductor substrate is a monocrystalline silicon substrateand the semiconductor body 208 is a single crystalline silicon-germaniumalloy. In an embodiment of the present invention, the silicon germaniumalloy comprises between 5–40% germanium and ideally approximatelybetween 15–25% germanium.

In an embodiment of the present invention, the bulk semiconductorsubstrate 202 is a monocrystalline silicon substrate and thesemiconductor body 208 is a silicon-carbon alloy.

In an embodiment of the present invention, semiconductor body 208 isformed to a thickness less than the amount at which the exteriorsurfaces of the semiconductor body 208 will cause relaxation in thecrystal lattice. In an embodiment of the present invention,semiconductor body 208 is formed to a thickness between 100–2000 Å andmore particularly between 200–1000 Å. In an embodiment of the presentinvention, the thickness and height of the semiconductor body 208 areapproximately the same.

In an embodiment of the present invention, the width of thesemiconductor body 208 is between half the body 208 height to two timesthe body 208 height. In an embodiment of the present invention,semiconductor body 208 is doped to a p type conductivity with aconcentration between 1×10¹⁶ to 1×10¹⁹ atoms/cm³ for an n typesemiconductor device and is doped to an n type conductivity with aconcentration between 1×10¹⁶ to 1×10¹⁹ atoms/cm³ for a p typesemiconductor device. In an embodiment of the present invention, thesemiconductor body 208 is intrinsic semiconductor, such as an undoped orintrinsic silicon film.

Transistor 200 includes a semiconductor capping layer 210 formed on thesidewalls 211 of semiconductor body 208 as well as on the top surface209 of semiconductor body 208. Semiconductor capping layer 210 is asingle crystalline semiconductor film. In an embodiment of the presentinvention, the semiconductor capping layer 210 is formed of asemiconductor material having a different lattice constant than thesemiconductor body 208 so that a strain is formed in the capping layer.In an embodiment of the present invention, the capping layer has atensile strain. A tensile strain is thought to improve the mobility ofelectrons. In an embodiment of the present invention, the capping layerhas a compressive strain. A compressive strain is thought to improvehole mobility. In an embodiment of the present invention, current flowsin a direction perpendicular to the strain in capping layer 210. In anembodiment of the present invention, the strain in the capping layer 210on the sidewalls 211 of semiconductor body 208 is greater than thestrain in the capping layer 210 on the top surface 209 of semiconductorbody 208.

In an embodiment of the present invention, the semiconductor cappinglayer 210 is a single crystalline silicon film. In an embodiment of thepresent invention, the capping layer 210 is a single crystalline siliconfilm formed on a silicon-germanium alloy body 208. A single crystallinesilicon film formed on a silicon-germanium alloy semiconductor body 208will cause the single crystalline silicon film to have a tensile stress.In an embodiment of the present invention, the capping layer 210 is asingle crystalline silicon film formed on a silicon-carbon alloysemiconductor body 208. A single crystalline silicon capping layer 210formed on a silicon-carbon alloy semiconductor body 208 will cause thesingle crystalline silicon film 210 to have a compressive stress.

In an embodiment of the present invention, the semiconductor cappinglayer 210 is formed to a thickness less than the amount at which thelattice of the single crystalline film will relax. In an embodiment ofthe present, the semiconductor capping layer 210 is formed to athickness between 50–300 Å. In an embodiment of the present invention,the thickness of the capping layer on the sidewalls 211 of semiconductorbody 208 is the same as the thickness of the capping layer 210 on thetop surface 209 of semiconductor body 208 as illustrated in FIG. 2. Inan embodiment of the present invention, the semiconductor capping layer210 is formed thicker on the top surface of the semiconductor body 208than on the sidewalls 211, such as shown, for example, in FIG. 4C.

Transistor 200 includes a gate dielectric layer 212. Gate dielectriclayer 212 is formed on capping layer 210 formed on the sidewalls 211 ofsemiconductor body 208 and is formed on semiconductor capping layer 210formed on the top surface 209 of semiconductor body 208. Gate dielectriclayer 210 can be any well known gate dielectric layer. In an embodimentof the present invention, the gate dielectric layer is a silicon dioxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or a silicon nitride (Si₃N₄)dielectric layer. In an embodiment of the present invention, the gatedielectric layer 212 is a silicon oxynitride film formed to a thicknessbetween 5–20 Å. In an embodiment of the present invention, the gatedielectric layer 212 is a high K gate dielectric layer, such as a metaloxide dielectric, such as but not limited to tantalum pentaoxide(Ta₂O₅), titanium oxide (TiO₂), hafnium oxide (HfO) and zirconium oxide(ZrO). Gate dielectric layer 212, however, can be other types of high Kdielectrics, such as but not limited to PZT and BST.

Transistor 200 includes a gate electrode 214. Gate electrode 214 isformed on and around the gate dielectric layer 212 as shown in FIG. 2.Gate electrode 214 is formed on and adjacent to gate dielectric layer212 formed on capping layer 210 formed on sidewall 211 of semiconductorbody 208 and is formed on gate dielectric layer 212 formed on cappinglayer 210 formed on the top surface 209 of semiconductor body 208 and isformed on or adjacent to gate dielectric layer 212 formed on cappinglayer 210 formed on sidewall 211 of gate electrode 208 as shown in FIG.2. Gate electrode 214 has a pair of laterally opposite sidewalls 216separated by a distance which defines the gate length (Lg) of transistor200. In an embodiment of the present invention, the laterally oppositesidewalls 216 of gate electrode 214 run in a direction perpendicular tothe laterally opposite sidewalls 211 of semiconductor body 208. Gateelectrode 214 can be formed of any suitable gate electrode material. Inan embodiment of the present invention, gate electrode 214 comprisespolycrystalline silicon film doped to a concentration density between1×10¹⁹ to 1×10²⁰ atoms/cm³. Gate electrode 214 can be doped to an n typeconductivity for an n type device and p type conductivity for a p typedevice. In an embodiment of the present invention, the gate electrodecan be a metal gate electrode. In an embodiment of the presentinvention, the gate electrode 214 is formed of a metal film having awork function which is tailored for an n type device, such as a workfunction between 3.9 eV to 4.2 eV. In an embodiment of the presentinvention, the gate electrode 214 is formed from a metal film having awork function tailored for a p type device, such as a work functionbetween 4.9 eV to 5.2 eV. In an embodiment of the present invention, thegate electrode 214 is formed from a material having midgap work functionbetween 4.6 to 4.8 eV. A midgap work function is ideal for use whensemiconductor body 208 and capping layer 210 are intrinsic semiconductorfilms. It is to be appreciated that gate electrode 214 need notnecessarily be a single material and can be composite stack of thinfilms, such as but not limited to polycrystalline silicon/metalelectrode or metal polycrystalline silicon electrode.

Transistor 200 has a pair source/drain regions formed in semiconductorbody 208 as well as in capping layer on opposite sides of a laterallyopposite sidewalls 216 of gate electrode 214 as shown in FIG. 2. Thesource/drain regions 218 are doped to an n type conductivity whenforming an n type device and doped to a p type conductivity when forminga p type device. In an embodiment of the present invention, thesource/drain regions have doping concentration of between 1×10¹⁹ to1×10²¹ atoms/cm³. The source/drain regions 218 can be formed of uniformconcentration or can include subregions of different concentrations ordoping profiles, such as tip regions (e.g., source/drain extensions). Inan embodiment of the present invention, when transistor 200 is asymmetrical transistor the source and drain regions will have the samedoping concentration profile. In an embodiment of the present invention,transistor 200 is an asymmetrical transistor, the source region anddrain region may vary in order to obtain particular electricalcharacteristics.

The portion of the semiconductor body 208 and capping layer 210 locatedbetween the source/drain regions 216 and beneath the gate electrode 214defines a channel region of the transistor. The channel region can alsobe defined as the area of semiconductor body 208 and capping layer 210surrounded by gate electrode 214. The source/drain regions typicallyextend slightly beneath the gate electrode through, for example,diffusion to define the channel region slightly smaller than the gateelectrode length (Lg). When transistor 300 is turned “ON” an inversionlayer is formed in the channel region of the device which forms aconductive channel which enables current to travel between thesource/drain region 340. The inversion layer or conductive channel formsin the surface of the capping layer on the sidewalls 211 ofsemiconductor body 208 as well as in the surface of capping layer 210 onthe top surface 209 of semiconductor body 208.

By providing a gate dielectric layer 212 and a gate electrode 214 whichsurrounds the semiconductor body 208 and capping layer 210 on threesides, the nonplanar transistor is characterized as having threechannels and three gates, one gate (G1) which extends between thesource/drain regions on one side 211 of semiconductor body 208, a secondgate (G2) which extends between the source/drain regions on the topsurface 209 of semiconductor body 208 and the third (G3) which extendsbetween the source/drain regions on sidewall 211 of semiconductor body208. The gate “width” (Gw) of transistor 200 is the sum of the width ofthe three channel regions. That is, the gate width of transistor 200 isequal to the height of semiconductor body 208 plus the thickness of thecapping layer on the top surface of sidewall 211, plus the width ofsemiconductor body 208 plus the thickness of the capping layer on eachof the sides 211 of semiconductor body plus the height of semiconductorbody 208 plus the thickness of capping layer 210 on the top surface 209of semiconductor body 208. Larger “width” transistor can be obtained byusing multiple semiconductor bodies 208 and capping layers surrounded bya single gate electrode, such as illustrated in FIG. 3I.

Although a tri-gate transistor 200 is illustrated in FIG. 2, the presentinvention is equally applicable to other nonplanar transistors. Forexample, the present invention is applicable to a “finfet” or a doublegate transistor or just two gates are formed on opposite sides of thesemiconductor body. Additionally, the present invention, is applicableto “omega” gates or wrap around gate devices where the gate electrodewraps around the semiconductor body as well as underneath a portion ofthe semiconductor body. Performance of “finfet” devices and “omega”devices can be improved by including a strained capping layer 210 formedon a semiconductor body 208 and thereby enhancing the mobility ofcarriers in the device. It is to be appreciated that a nonplanar deviceis a device which when turned “ON” forms a conductive channel or aportion of the conductive channel in a direction perpendicular to theplane of the substrate 202. A nonplanar transistor can also be said tobe a device where the conductive channel regions are formed both in thehorizontal and vertical directions.

FIGS. 3A–3I illustrate a method of forming a bulk nonplanar transistorhaving a strain enhanced mobility in accordance with an embodiment ofthe present invention. First a semiconductor substrate 300 is providedas shown in FIG. 3A. In an embodiment of the present invention,semiconductor substrate 300 is a monocrystalline silicon substrate.Substrate 300 need not necessarily be a silicon substrate and can beother types of substrates, such as a silicon germanium substrate, agermanium substrate, a silicon germanium alloy, a gallium arsenide,InSb, and GaP. In an embodiment of the present invention, thesemiconductor substrate 300 is an intrinsic (i.e., undoped) siliconsubstrate. In other embodiments of the present invention, thesemiconductor substrate 300 is doped to a p type or n type conductivitywith a concentration between 1×10¹⁶ to 1×10¹⁹ atom/cm³. Next, a maskhaving mask portions 302 for forming isolation regions is formed onsubstrate 300 as shown in FIG. 3A. In an embodiment of the presentinvention, the mask is an oxidation resistant mask. In an embodiment ofthe present invention, the mask portions 302 comprise a thin pad oxidelayer 304 and a thicker silicon nitride or oxidation resistant layer306. The mask portions 302 define active regions 308 in substrate 300where transistor bodies are to be formed. The mask portions 302 can beformed by blanket depositing a pad oxide layer and then a siliconnitride layer over substrate 300. Next, well known photolithographytechniques are used to mask, expose and develop a photoresist maskinglayer over locations where mask portions 302 are to be formed. Thenitride film 306 and the pad oxide layers 304 are then etched inalignment with the formed photoresist mask to form mask portions 302 asshown in FIG. 3A.

In an embodiment of the present invention, mask portions 302 have awidth (W1) which is the minimum width or minimum feature dimension(i.e., critical dimension (CD)) which can be defined utilizingphotolithography in the fabrication of the transistor. Additionally, inan embodiment of the present invention, mask portions 302 are separatedby a distance D1 which is the minimum distance which can be definedutilizing photolithography in the fabrication process. That is, maskportions 302 have the smallest dimension and are spaced apart by thesmallest dimension (i.e., critical dimensions) which can be reliably andachieved utilizing the photolithography process used to fabricate thetransistor. In this way, mask portions 302 are defined to have thesmallest size and greatest density capable of being achieved with thephotolithography process used in fabrication of the transistor.

In an embodiment of the present invention, mask portions 302 have athickness (T1) which is equal to or greater than the thickness or heightdesired for the subsequently formed semiconductor body or bodies.

Next, as shown in FIG. 3B, the exposed portions of semiconductor 300 areetched in alignment with the outside edges of mask portion 302 to formtrench openings 310. The trench openings are etched to a depthsufficient to isolate adjacent transistors from one another.

Next, as shown in FIG. 3C, the trenches are filled with a dielectriclayer 312 to form shallow trench isolation (STI) regions 312 insubstrate 300. In an embodiment of the present invention, the dielectriclayer is formed by first growing a thin liner oxide in the bottom ofsidewalls of trench 310. Next, trench 312 is filled by blanketdepositing an oxide dielectric layer over the liner oxide by, forexample, a high density plasma (HDP) chemical vapor deposition process.The fill dielectric layer will also form on the top of mask portions302. The fill dielectric layer can then be removed from the top of maskportions 302 by, for example, chemical mechanical polishing. Thechemical mechanical polishing process is continued until the top surfaceof mask portions 302 is revealed and the top surface of shallow trenchisolation regions 312 substantially planar with the top surface of maskportion 302 as shown in FIG. 3C.

Although shallow trench isolation regions are ideally used in thepresent invention, other well known isolation regions and techniques,such as local oxidation of silicon (LOCOS) or recessed LOCOS may beutilized.

Next, as shown in FIG. 3D, mask portions 302 are removed from substrate300 to form semiconductor body openings 314. First a silicon nitrideportion 306 is removed utilizing an etchant which etches away theoxidation resistant or silicon nitride portion 306 without significantlyetching the isolation regions 312. After removing silicon nitrideportion 306, the pad oxide portion 304 is removed. Pad oxide portion 304can be removed, for example, with a wet etchant comprising hydrofluoricacid (HF). Removing of mask portions 302 forms a semiconductor bodyopening or trench 314 having substantially vertical sidewalls. Thevertical sidewall enables the semiconductor body to be grown within thetrench and confined therein to enable a semiconductor body to be formedwith nearly vertical sidewalls.

Next, as shown in FIG. 3E, a semiconductor body film 316 is formed inopening 314 as shown in FIG. 3E. In an embodiment of the presentinvention, the semiconductor body film 316 is an epitaxial semiconductorfilm. In an embodiment of the present invention, when a strain enhancedsemiconductor device is desired, the semiconductor film is formed from asingle crystalline semiconductor film having a different latticeconstant or different lattice size than the underlying semiconductorsubstrate upon which it is grown, so that the semiconductor film isunder strain. In an embodiment of the present invention, the singlecrystalline silicon film 316 has a larger lattice constant or latticesize than the underlying semiconductor substrate 300. In an embodimentof the present invention, the single crystalline semiconductor film 316has a smaller lattice size or constant than the underlying semiconductorsubstrate 300.

In an embodiment of the present invention, the semiconductor film 316 isan epitaxial silicon germanium alloy film selectively grown on a siliconmonocrystalline substrate 300. A silicon germanium alloy can beselectively grown in an epitaxial reactor utilizing a deposition gascomprising, dichlorosilance (DCS), H₂, germane (GeH₄), and HCl. In anembodiment of the present invention, the silicon germanium alloycomprises between 5–40% germanium and ideally between 15–25% germanium.In an embodiment of the present invention, epitaxial semiconductor film316 is a single crystalline silicon carbon alloy formed on a siliconsubstrate 300. The single crystalline semiconductor film 316 isdeposited to a thickness desired for the thickness of the semiconductorbody. In an embodiment of the present invention, it is grown ordeposited to a thickness less than the height of the top surface ofisolation regions 312. In this way, the isolation regions 312 confinesthe semiconductor film 316 within the trench so that a semiconductorfilm with nearly vertical sidewalls is formed. Alternatively,semiconductor film 316 can be blanket deposited over substrate 300including within trench 314 and on top of isolation regions 312 and thenpolished back so that the semiconductor film 316 is removed from the topof the isolation regions and remains only within trenches 314 as shownin FIG. 3E.

In an embodiment of the present invention, the semiconductor film 316 isan undoped or intrinsic semiconductor film. In an embodiment of thepresent invention, when fabricating a p type device, the semiconductorfilm 316 doped to an n type conductivity with a concentration between1×10¹⁶ to 1×10¹⁹ atoms/cm³. In an embodiment of the present invention,when fabricating an n type device the semiconductor film 316 is doped toa p type conductivity with a concentration between 1×10¹⁶ to 1×10¹⁹atoms/cm³. The semiconductor film 316 can be doped during deposition inan “insitu” process by including a dopant gas in the deposition processgas mix. Alternatively, the semiconductor film 316 can be subsequentlydoped by, for example, ion implantation or thermal diffusion to form adoped semiconductor film 316.

Next, isolation regions 312 are etched back or recessed to expose thesidewalls 320 of semiconductor film 316 and thereby form semiconductorbodies 318 as shown in FIG. 3F. Semiconductor bodies 318 have nearlyvertical sidewalls 320 because semiconductor film 316 was laterallyconfined by isolation regions 312 during deposition. Isolation regions312 are etched back with an etchant which does not significantly etchthe semiconductor film 316. When semiconductor film 316 is a silicon orsilicon alloy isolation regions 312 can be recessed utilizing a wetetchant comprising HF. In an embodiment of the present invention,isolation regions are etched back to a level so that they aresubstantially planar with the top surface of the active regions 308formed in semiconductor substrate 300 as shown in FIG. 3F.

Next, as shown in FIG. 3G, a semiconductor capping layer 322 is formedon the top surface 319 and sidewalls 320 of semiconductor body 318.Semiconductor capping layer 322 is a single crystalline semiconductorfilm. In an embodiment of the present invention, the semiconductorcapping layer 322 is formed of a material having a different latticeconstant or size than semiconductor body 318. In an embodiment of thepresent invention, semiconductor capping layer 322 is a singlecrystalline silicon film. In an embodiment of the present invention,semiconductor capping layer 322 is a single crystalline silicon filmformed on a silicon germanium alloy body 318. In an embodiment of thepresent invention, semiconductor capping layer 322 is a singlecrystalline silicon film formed on a silicon-carbon alloy semiconductorbody 318. A single crystalline silicon capping layer 322 can beselectively deposited in an epitaxial deposition reactor utilizing aprocess gas comprising DCS, HCl and H₂. In an embodiment of the presentinvention, semiconductor capping layer 322 is formed to a thickness lessthan an amount which will cause substantial relaxation in semiconductorcapping layer 322. In an embodiment of the present invention,semiconductor capping layer 322 is formed to a thickness sufficient toenable the entire inversion layer to be formed in the capping layer whenthe transistor is turn “ON”. In an embodiment of the present invention,semiconductor capping layer 322 is formed to a thickness between 50–300Å. In an embodiment of the present invention, semiconductor cappinglayer 322 is an undoped or intrinsic semiconductor film. In anembodiment of the present invention, semiconductor capping layer 322 isdoped to an n type conductivity between 1×10¹⁶ to 1×10¹⁹ atoms/cm³ whenforming a p type device and is doped to a p type conductivity between1×10¹⁶ to 1×10¹⁹ atoms/cm³ when forming an n type device. In anembodiment of the present invention, semiconductor capping layer 322 isdoped in an insitu deposition process. Alternatively, capping layer 322can be doped by other well known techniques, such as by ion implantationor solid source diffusion.

Next, as shown in FIG. 3H, a gate dielectric film 324 is formed oncapping layer 322 formed on the sidewalls 320 of semiconductor body 318and is formed on the capping layer 322 formed on the top surface 319 ofsemiconductor body 318 as shown in FIG. 3H. In an embodiment of thepresent invention, gate dielectric layer 324 is a grown gate dielectriclayer, such as but not limited to a silicon dioxide layer, a siliconoxynitride layer or a combination thereof. A silicon oxide or siliconoxynitride layer can be grown on semiconductor capping layer utilizing awell known dry/wet oxidation process. When gate dielectric layer 324 isgrown it will form only on semiconductor containing areas, such ascapping layer 322 and not on isolation regions 312. Alternatively, gatedielectric layer 324 can be a deposited dielectric layer. In anembodiment of the present invention, gate dielectric layer 324 is a highK gate dielectric layer, such as a metal oxide dielectric layer, such asbut not limited to hafnium oxide, zirconium oxide, tantalum oxide andtitanium oxide. A high K metal oxide dielectric layer can be depositedby any well known technique, such as chemical vapor deposition orsputter deposition. When gate dielectric layer 324 is deposited it willalso form on isolation regions 312.

Next, as shown in FIG. 3H, a gate electrode material 326 is blanketdeposited over substrate 300 so that it deposits onto and around gatedielectric layer 324. That is, the gate electrode material is depositedonto the gate dielectric layer 324 formed on capping layer 322 formed onthe top surface of semiconductor body 318 and is formed or adjacent tocapping layer 322 formed on the sidewalls 320 of semiconductor body 318.In an embodiment of the present invention, the gate electrode material326 is polycrystalline silicon. In an embodiment of the presentinvention, the gate electrode material 326 is a metal film. In anembodiment of the present invention, gate electrode material 326 is ametal film having a work function tailored for an n type device and inan embodiment of the present invention, the gate electrode material ismetal film having a work function tailored for a p type device. Gateelectrode material 326 is formed to a thickness sufficient to completelycover or surround semiconductor bodies 318, capping layer 322 and gatedielectric layer 324 as shown in FIG. 3H.

Next, as shown in FIG. 3I, the gate electrode material 326 and gatedielectric layer 324 are patterned by well known techniques to form agate electrode 330 and a gate dielectric layer 328. Gate electrodematerial 326 and gate dielectric layer 324 can be patterned utilizingwell known photolithography and etching techniques. Gate electrode 330has a pair of laterally opposite sidewalls 332 which define the gatelength of the device. In an embodiment of the present invention,laterally opposite sidewalls 332 run in a direction perpendicular tosemiconductor bodies 318. Although, a subtractive process is shown forthe formation of gate electrode 330, other well known techniques, suchas a replacement gate process may be utilized to form gate electrode330.

Next, as also shown in FIG. 3I, a pair of source/drain regions 340 areformed in capping layer 332 and semiconductor body 318 on opposite sidesof gate electrode 330. When forming an n type device, source/drainregions can be formed to an n type conductivity with a concentrationbetween 1×10²⁰ to 1×10²¹ atoms/cm³. In an embodiment of the presentinvention, when forming a p type device, source/drain regions having a ptype conductivity with a concentration between 1×10²⁰ to 1×10²¹atoms/cm³ can be formed. Any well known technique, such as ionimplantation or thermal diffusion, may be utilized to form thesource/drain regions. When ion implantation is used, the gate electrode330 can be used to mask the channel region of the transistor from theion implantation process and thereby self-aligning the source/drainregions 340 with the gate electrode 330. Additionally, if desired,source/drain regions may include sub-regions, such as source/drainextensions and source/drain contact regions. Well known processesincluding formation of spacers can be utilized to form the sub-regions.Additionally, if desired, silicide can be formed on the source/drainregions 340 and on top of the gate electrode 330 to further decrease theelectrical contact resistance. This completes the fabrication of bulknonplanar transistor having strain enhanced mobility.

Well known “back end” techniques can be utilized to form metal contacts,metallization layers and interlayer dielectrics to interconnect varioustransistors together into functional integrated circuits, such asmicroprocessors.

A valuable aspect of the present invention, is that the capping layerincreases the gate width of the transistor. In this way, minimum featuredimension and spacing can be used to form the semiconductor bodies andthen the capping layer can be formed on and around the minimally definedsemiconductor bodies to increase the gate width of the device. Thisincreases the current per area of the device which improves deviceperformance. Formation of a capping layer on minimally defined andseparated features reduces the distance between minimally spaced bodiesto a distance less than the critical dimension or less than thedimension achievable with photolithography process used to define thedevice. In this way, the formation of a capping layer enables largergate width to be achieved with each semiconductor body while stilldefining the bodies with the minimum critical dimensions (CD) andspacing. Utilizing a capping layer to increase the gate width isvaluable even in applications which do not require or desire stressenhanced mobility. As such, embodiments of the present invention includeapplications where, for example, silicon capping layers are formed onminimally spaced silicon bodies in order to increase the gate width ofthe fabricated transistor. Additionally, use of a capping layer toincrease gate width per area is useful in non-bulk devices, such astri-gate or nonplanar devices formed on insulated substrates, such as insilicon on insulator (SOI) substrates.

In embodiments of the present invention, stacks of semiconductor films(i.e., bulk semiconductor 300, semiconductor body 318 and capping layer322) are engineered to produce high strain in the capping layer 322which can dramatically increase carrier mobility. FIG. 5 illustrates howa bulk silicon monocrystalline silicon substrate, a silicon germaniumalloy semiconductor body 320 and a silicon capping layer 322 can producehigh tensile stress in the silicon capping layer 322. When growing anepitaxial silicon germanium alloy film 316 on a monocrystallinesubstrate 300 (FIG. 3E) the lattice constant of the plane 502 of thesilicon germanium film 318 parallel to the surface of the siliconmonocrystalline substrate 300 is matched to the silicon lattice of thebulk silicon substrate 300. The lattice constant of the plane 504 of thesilicon germanium alloy 316 perpendicular to the silicon substratesurface is larger than the plane 502 parallel to the silicon substrate300 due to the tetragonal distortion of the silicon germanium epitaxialfilm 316. Once the isolation regions 312 are recessed (FIG. 3F) to formsilicon germanium body 318 the silicon germanium lattice on the top 319will expand and the lattice constant on the sides will contract due tothe presence of free surface. In general the lattice constant on thesidewall 320 of the silicon germanium alloy 318 will be larger than thelattice constant on the top surface 319 of the silicon germanium alloywhich will be greater than the lattice constant of the silicon germaniumalloy on the silicon monocrystalline substrate. When a silicon cappinglayer 322 is grown on the strained silicon germanium alloy, (FIG. 3G)the silicon germanium alloy 318 will impose its lengthened vertical celldimension 504 on an already smaller cell dimension of the siliconcapping layer 322 producing a orthorhombic strained silicon cappinglayer 322 on the sidewalls of the SiGe body 318. Thus, the siliconcapping layer formed on the sidewalls 322 of the silicon germanium alloywill witness a substantial tensile strain and a lower but significanttensile strain on the top surface 319 of the silicon germanium alloy.The strain produced in silicon capping layer 322 is in a directionperpendicular to current flow in the device.

FIGS. 4A–4C illustrate a method of forming a bulk nonplanar transistorhaving strain enhanced mobility wherein the capping layer is formedthicker on the top surface of the semiconductor body than on thesidewalls. As illustrated in FIG. 4A, semiconductor body film 316 isgrown between isolation regions 312 as described with respect to FIG.3E. In this embodiment, however, a first portion 410 of the cappinglayer is grown on semiconductor body 316 prior to recessing isolationregions 312. In an embodiment of the present invention, silicon nitridelayer 306 is formed thicker than necessary for the semiconductor body318 so that additional room is provided to enable the first portion 410of the semiconductor capping layer to be grown within the trench 310. Inthis way, the first portion of the capping layer 410 can be confinedwithin the isolation regions 312. After formation of the first portion410 of the capping layer, the isolation regions 312 are recessed back asdescribed above to form a semiconductor body 318 having a capping layer410 formed on the top surface thereof as shown in FIG. 4B. Next, asshown in FIG. 4C, a second portion 412 of the capping layer is grown onthe sidewalls 320 of the semiconductor body 318 and on the first portion410 of the capping layer formed on the top surface 319 of semiconductorbody 320. In an embodiment of the present invention, the semiconductorcapping layer 410 is formed to a thickness substantially equal to thethickness of the second portion of the capping layer 412. In this way,when a substantially square semiconductor body 318 is formed, thesemiconductor body 318 plus capping layer will still provide asubstantially square capped body. Next, processing can continue asillustrated in FIGS. 3H and 3I to complete fabrication of the bulknonplanar transistor having a strain enhanced mobility.

1. A semiconductor device comprising: a semiconductor body on an activeregion of a bulk semiconductor substrate, said semiconductor body havinga top surface and laterally opposite sidewalls; an isolation region onsaid bulk semiconductor substrate, said isolation region adjacent tosaid active region; a semiconductor capping layer formed on the topsurface and on the sidewalls of said semiconductor body, wherein saidsemiconductor capping layer is formed thicker on the top surface of saidsilicon germanium body than on the sidewalls of said semiconductor body;a gate dielectric layer formed on said semiconductor capping layer onsaid top surface and on said sidewalls of said semiconductor body; agate electrode having a pair of laterally opposite sidewalls formed onand around said gate dielectric layer; and a pair of source/drainregions formed in said semiconductor body on opposite sides of said gateelectrode.
 2. The semiconductor device of claim 1 wherein saidsemiconductor capping layer has a tensile stress.
 3. The semiconductordevice of claim 2 wherein said semiconductor capping layer has greatertensile stress on the sidewalls of said semiconductor body than on thetop surface of said semiconductor body.
 4. The semiconductor device ofclaim 2 wherein said source/drain regions are n type conductivity. 5.The semiconductor device of claim 1 wherein said bulk semiconductorsubstrate is a silicon substrate, wherein said semiconductor body is asilicon germanium alloy and wherein said semiconductor capping layer isa silicon film.
 6. The semiconductor device of claim 1 wherein saidsemiconductor capping layer has a compressive stress.
 7. Thesemiconductor device of claim 6 wherein said semiconductor capping layerhas a greater compressive stress on the sidewalls than on the topsurface of said semiconductor body.
 8. A semiconductor devicecomprising: a silicon germanium body formed on an active region of abulk silicon monocrystalline substrate, said silicon germanium bodyhaving a top surface and a pair of laterally opposite sidewalls; anisolation region on said bulk silicon monocrystalline substrate, saidisolation region adjacent to said active region and adjacent to aportion of said silicon germanium body; a silicon film formed on saidtop surface and on said sidewalls of said silicon germanium body,wherein said silicon film is formed thicker on the top surface of saidsilicon germanium body than on the sidewalls of said silicon germaniumbody; a gate dielectric layer formed on said silicon film on said topsurface of said silicon germanium body and on said silicon film on saidsidewalls of said silicon germanium body; a gate electrode having a pairof laterally opposite sidewalls formed on and around said gatedielectric layer; and a pair of source/drain regions formed in saidsilicon germanium body on opposite sides of said gate electrode.
 9. Thesemiconductor device of claim 8 wherein said silicon film has athickness between 50–300 Å.
 10. The semiconductor device of claim 8wherein said silicon germanium alloy comprises between 5–40% germanium.11. The semiconductor device of claim 10 wherein said silicon germaniumalloy comprises approximately 15–25% germanium.
 12. The semiconductordevice of claim 8 wherein said source/drain regions are n typeconductivity.
 13. A semiconductor device comprising: a silicon-carbonalloy body formed on an active region of a silicon monocrystallinesubstrate, said silicon carbon alloy body having a top surface and apair of laterally opposite sidewalls; an isolation region on said bulksemiconductor substrate, said isolation region adjacent to said activeregion and adjacent to a portion of said silicon-carbon alloy body; asilicon film formed on said top surface and on said sidewalls of saidsilicon carbon alloy body, wherein said silicon film is formed thickeron the top surface of said silicon carbon alloy body than on thesidewalls of said silicon germanium body; a gate dielectric layer formedon said silicon film on said top surface of said silicon-carbon body andon said silicon film on said sidewalls of said silicon-carbon alloybody; a gate electrode having a pair of laterally opposite sidewallsformed on and around said gate dielectric layer; and a pair ofsource/drain regions formed in said silicon carbon-alloy body onopposite sides of said gate electrode.
 14. The semiconductor device ofclaim 13 wherein said silicon film is formed to a thickness between50–300 Å.
 15. The semiconductor device of claim 14 wherein said siliconfilm has a thickness between 50–300 Å.
 16. The semiconductor device ofclaim 13 wherein said source/drain regions are p type conductivity. 17.A semiconductor device comprising: a silicon germanium body formed on asilicon monocrystalline substrate, said silicon germanium body having atop surface and a pair of laterally opposite sidewalls; a silicon filmformed on said top surface and on said sidewalls of said silicongermanium body, wherein said silicon film is formed thicker on the topsurface of said silicon germanium body than on the sidewalls of saidsilicon germanium body; a gate dielectric layer formed on said siliconfilm on said top surface of said silicon germanium body and on saidsilicon film on said sidewalls of said silicon germanium body; a gateelectrode having a pair of laterally opposite sidewalls formed on andaround said gate dielectric layer; and a pair of source/drain regionsformed in said silicon germanium body on opposite sides of said gateelectrode.
 18. A semiconductor device comprising: a semiconductor bodyon a bulk semiconductor substrate, said semiconductor body having a topsurface and laterally opposite sidewalls, wherein said semiconductorbody is comprised of a material which has a different lattice constantthan said bulk semiconductor substrate; semiconductor capping layerformed on the top surface and on the sidewalls of said semiconductorbody, wherein said semiconductor capping layer comprises a materialwhich has a different lattice constant than said semiconductor body andwherein said semiconductor capping layer is formed thicker on the topsurface of said semiconductor body than on the sidewalls of saidsemiconductor body; a gate dielectric layer formed on said semiconductorcapping layer on said top surface and on said sidewalls of saidsemiconductor body; a gate electrode having a pair of laterally oppositesidewalls formed on and around said gate dielectric layer; and a pair ofsource/drain regions formed in said semiconductor body on opposite sidesof said gate electrode.
 19. The semiconductor device of claim 18,wherein said semiconductor body comprises a semiconductor materialhaving a lattice constant that is larger than the lattice constant ofsaid bulk semiconductor substrate.
 20. The semiconductor device of claim18, wherein said semiconductor body comprises a semiconductor materialhaving a lattice constant that is smaller than the lattice constant ofsaid bulk semiconductor substrate.
 21. The semiconductor device of claim18, wherein said semiconductor capping layer comprises a semiconductormaterial having a lattice constant that is larger than the latticeconstant of said semiconductor body.
 22. The semiconductor device ofclaim 18, wherein said semiconductor capping layer comprises asemiconductor material having a lattice constant that is smaller thanthe lattice constant of said semiconductor body.
 23. The semiconductordevice of claim 18, wherein said bulk semiconductor substrate is asilicon substrate, wherein said semiconductor body is a silicongermanium alloy and wherein said semiconductor capping layer is asilicon film.
 24. The semiconductor device of claim 18, wherein saidbulk semiconductor substrate is a silicon substrate, wherein saidsemiconductor body is a silicon-carbon alloy body, and wherein saidsemiconductor capping layer is a silicon capping layer.
 25. Asemiconductor device comprising: a semiconductor body on a bulksemiconductor substrate, said semiconductor body having a top surfaceand laterally opposite sidewalls; a semiconductor capping layer on thetop surface and on the sidewalls of said semiconductor body, whereinsaid semiconductor capping layer is thicker on the top surface of saidsemiconductor body than on the sidewalls of said semiconductor body; agate dielectric layer on said semiconductor capping layer on said topsurface and on said sidewalls of said semiconductor body; a gateelectrode having a pair of laterally opposite sidewalls on and aroundsaid gate dielectric layer; and a pair of source/drain regions in saidsemiconductor body on opposite sides of said gate electrode.